Glass-like Transport Dominates Ultralow Lattice Thermal Conductivity in Modular Crystalline Bi4O4SeCl2

Crystalline Bi4O4SeCl2 exhibits record-low 0.1 W/mK lattice thermal conductivity (κL), but the underlying transport mechanism is not yet understood. Using a theoretical framework which incorporates first-principles anharmonic lattice dynamics into a unified heat transport theory, we compute both the particle-like and glass-like components of κL in crystalline and pellet Bi4O4SeCl2 forms. The model includes intrinsic three- and four-phonon scattering processes and extrinsic defect and extended defect scattering contributing to the phonon lifetime, as well as temperature-dependent interatomic force constants linked to phonon frequency shifts and anharmonicity. Bi4O4SeCl2 displays strongly anisotropic complex crystal behavior with dominant glass-like transport along the cross-plane direction. The uncovered origin of κL underscores an intrinsic approach for designing extremely low κL materials.

Impact of Local Composition on the Emission Spectra of InGaN Quantum-Dot LEDs

A possible solution for the realization of high-efficiency visible light-emitting diodes (LEDs) exploits InGaN-quantum-dot-based active regions. However, the role of local composition fluctuations inside the quantum dots and their effect of the device characteristics have not yet been examined in sufficient detail. Here, we present numerical simulations of a quantum-dot structure restored from an experimental high-resolution transmission electron microscopy image. A single InGaN island with the size of ten nanometers and nonuniform indium content distribution is analyzed. A number of two- and three-dimensional models of the quantum dot are derived from the experimental image by a special numerical algorithm, which enables electromechanical, continuum k→·p→, and empirical tight-binding calculations, including emission spectra prediction. Effectiveness of continuous and atomistic approaches are compared, and the impact of InGaN composition fluctuations on the ground-state electron and hole wave functions and quantum dot emission spectrum is analyzed in detail. Finally, comparison of the predicted spectrum with the experimental one is performed to assess the applicability of various simulation approaches.

Predicting the Lattice Thermal Conductivity in Nitride Perovskite LaWN3 from ab initio Lattice Dynamics

Using a density functional theory-based thermal transport model, which includes the effects of temperature (T)-dependent potential energy surface, lattice thermal expansion, force constant renormalization, and higher-order quartic phonon scattering processes, it is found that the recently synthesized nitride perovskite LaWN₃ displays strong anharmonic lattice dynamics manifested into a low lattice thermal conductivity (κ_L ) and a non-standard κ_L ∝T^-0.491 dependence. At high T, the departure from the standard κ_L ∝T^-1 law originates in the dual particle-wave behavior of the heat carrying phonons, which includes vibrations tied to the N atoms. While the room temperature κ_L =2.98 W mK^-1 arises mainly from the conventional particle-like propagation of phonons, there is also a significant atypical wave-like phonon tunneling effect, leading to a 20% glass-like heat transport contribution. The phonon broadening effect lowers the particle-like contribution but increases the glass-like one. Upon T increase, the glass-like contribution increases and dominates above T = 850 K. Overall, the low κ_L with a weak T-dependence points to a new utility for LaWN₃ in energy technology applications, and motivates synthesis and exploration of nitride perovskites.

Four-phonon and electron-phonon scattering effects on thermal properties in two-dimensional 2H-TaS2

Thermal transport characteristics of monolayer trigonal prismatic tantalum disulfide (2H-TaS₂) are investigated using first-principles calculations combined with the Boltzmann transport equation. Due to a large acoustic-optical phonon gap of 1.85 THz, the four-phonon (4ph) scattering significantly reduces the room-temperature phononic thermal conductivity (κ_ph). With the further inclusion of phonon-electron scattering, κ_ph reduces to 1.78 W mK^-1. Nevertheless, the total thermal conductivity (κ_total) of 7.82 W mK^-1 is dominated by the electronic thermal conductivity (κ_e) of 6.04 W mK^-1. Due to the electron-phonon coupling, κ_e differs from the typical estimation based on the Wiedemann-Franz law with a constant Sommerfeld value. This work provides new insights into the physical mechanisms for thermal transport in metallic 2D systems with strong anharmonic and electron-phonon coupling effects. The phonon scattering beyond three-phonon (3ph) scattering and even κ_e are typically overlooked in computations, and the constant Sommerfeld value is widely used for separating κ_e and κ_ph from the experimental thermal conductivity. These conclusions have implications for both the computational and experimental measurements of the thermal properties of transition metal dichalcogenides.

Anisotropic Phononic and Electronic Thermal Transport in BeN4

Beryllium polynitride (BeN₄) has been recently synthesized under high-pressure conditions [Bykov et al. Phys. Rev. Lett. 2021, 126, 175501]. Its anisotropic lattice structure dependent on the applied pressure motivates exploration of its thermal transport properties with a theoretical framework that combines the Boltzmann transport equation with ab initio calculations. The bonding anisotropy (impacting the phonon and electron group velocities) and bonding anharmonicity (captured through three- and four-phonon scatterings) are reflected in the strong anisotropy of both phononic and electronic components of the thermal conductivity. Moreover, the pressure-driven evolution of the interlayer Be-N bonding, from partially covalent (under high-pressure synthesis conditions) to van der Waals (under ambient pressure), drives a largely interlayer thermal conductivity. These findings highlight an alternative strategy for achieving directional control of the thermal transport in synthetic materials.

Role of Phase Nanosegregation in the Photoluminescence Spectra of Halide Perovskites

The study of MAPbI₃ phase transitions based on temperature-dependent optical spectroscopy has recently gained a huge attention. Photoluminescence (PL) investigations of the tetragonal-orthorhombic transition suggest that tetragonal nanodomains are present below the transition temperature and signatures associated with tetragonal segregations are observed. We have studied the impact of phase nanosegregation across the orthorhombic-tetragonal phase transition of MAPbI₃ on the system's properties employing a tight binding (TB) approach. The particle swarm optimization has been used to obtain a consistent set of TB parameters, where the target properties of the system have been derived by first-principles calculations. The theoretical results have been compared with the measured PL spectra for a temperature range going from 10 to 100 K. Our model effectively captures the carriers' localization phenomenon induced by the presence of residual tetragonal nanodomains and demonstrates that the assumption of phase nanosegregation can explain the low-energy features in the PL spectra of MAPbI₃.

Phononic Thermal Transport along Graphene Grain Boundaries: A Hidden Vulnerability

While graphene grain boundaries (GBs) are well characterized experimentally, their influence on transport properties is less understood. As revealed here, phononic thermal transport is vulnerable to GBs even when they are ultra-narrow and aligned along the temperature gradient direction. Non-equilibrium molecular dynamics simulations uncover large reductions in the phononic thermal conductivity (κp ) along linear GBs comprising periodically repeating pentagon-heptagon dislocations. Green's function calculations and spectral energy density analysis indicate that the origin of the κp reduction is hidden in the periodic GB strain field, which behaves as a reflective diffraction grating with either diffuse or specular phonon reflections, and represents a source of anharmonic phonon-phonon scattering. The non-monotonic dependence with dislocation density of κp uncovered here is unaccounted for by the classical Klemens theory. It can help identify GB structures that can best preserve the integrity of the phononic transport.

DFTB+, a software package for efficient approximate density functional theory based atomistic simulations

DFTB+ is a versatile community developed open source software package offering fast and efficient methods for carrying out atomistic quantum mechanical simulations. By implementing various methods approximating density functional theory (DFT), such as the density functional based tight binding (DFTB) and the extended tight binding method, it enables simulations of large systems and long timescales with reasonable accuracy while being considerably faster for typical simulations than the respective ab initio methods. Based on the DFTB framework, it additionally offers approximated versions of various DFT extensions including hybrid functionals, time dependent formalism for treating excited systems, electron transport using non-equilibrium Green's functions, and many more. DFTB+ can be used as a user-friendly standalone application in addition to being embedded into other software packages as a library or acting as a calculation-server accessed by socket communication. We give an overview of the recently developed capabilities of the DFTB+ code, demonstrating with a few use case examples, discuss the strengths and weaknesses of the various features, and also discuss on-going developments and possible future perspectives.

Titanium-carbide MXenes for work function and interface engineering in perovskite solar cells

To improve the efficiency of perovskite solar cells, careful device design and tailored interface engineering are needed to enhance optoelectronic properties and the charge extraction process at the selective electrodes. Here, we use two-dimensional transition metal carbides (MXene Ti₃C₂T_x) with various termination groups (T_x) to tune the work function (WF) of the perovskite absorber and the TiO₂ electron transport layer (ETL), and to engineer the perovskite/ETL interface. Ultraviolet photoemission spectroscopy measurements and density functional theory calculations show that the addition of Ti₃C₂T_x to halide perovskite and TiO₂ layers permits the tuning of the materials' WFs without affecting other electronic properties. Moreover, the dipole induced by the Ti₃C₂T_x at the perovskite/ETL interface can be used to change the band alignment between these layers. The combined action of WF tuning and interface engineering can lead to substantial performance improvements in MXene-modified perovskite solar cells, as shown by the 26% increase of power conversion efficiency and hysteresis reduction with respect to reference cells without MXene.

Titanium-carbide MXenes for work function and interface engineering in perovskite solar cells

To improve the efficiency of perovskite solar cells, careful device design and tailored interface engineering are needed to enhance optoelectronic properties and the charge extraction process at the selective electrodes. Here, we use two-dimensional transition metal carbides (MXene Ti₃C₂T_x) with various termination groups (T_x) to tune the work function (WF) of the perovskite absorber and the TiO₂ electron transport layer (ETL), and to engineer the perovskite/ETL interface. Ultraviolet photoemission spectroscopy measurements and density functional theory calculations show that the addition of Ti₃C₂T_x to halide perovskite and TiO₂ layers permits the tuning of the materials' WFs without affecting other electronic properties. Moreover, the dipole induced by the Ti₃C₂T_x at the perovskite/ETL interface can be used to change the band alignment between these layers. The combined action of WF tuning and interface engineering can lead to substantial performance improvements in MXene-modified perovskite solar cells, as shown by the 26% increase of power conversion efficiency and hysteresis reduction with respect to reference cells without MXene.

Nanoscale morphology and electronic coupling at the interface between indium tin oxide and organic molecular materials

The correlation between nanoscale morphology and charge injection rates at the interface between an organic semiconductor layer and a transparent metal oxide electrode was investigated by integrating molecular dynamics simulations with electronic structure calculations. The simulation approach proposed has been applied to the analysis of the hole injection mechanism at the interface between an amorphous layer of tris[(3-phenyl-1H-benzimidazol-1-yl-2(3H)-ylidene)-1,2-phenylene]Ir (DPBIC), a hole transport and emitter molecule, and the surface of indium tin oxide (ITO), a material commonly used as anode in OLEDs. The link between interface morphology and charge injection was investigated by implementing a two-step, top-down simulation approach. Namely, nanoscale molecular aggregation phenomena at the organic/electrode interface were first assessed by molecular dynamics simulations, mimicking different processing conditions, and followed by density functional theory calculations of the electronic coupling between molecular levels and the manifold of electrode states involved in the charge injection process. The correlation between structural parameters and electronic coupling suggests a significant role of specific molecule/electrode configurations on charge transport processes at the interface, resulting in a broad distribution of charge injection rates, and highlights the link between molecular structure, nanoscale aggregation and processing in the realization of heterointerfaces for efficient charge injection in organic electronic devices.

Spatial and orientational dependence of electron transfer parameters in aggregates of iridium-containing host materials for OLEDs: coupling constrained density functional theory with molecular dynamics

The integration between molecular dynamics and constrained density functional theory allows to evaluate charge transport parameters in bulk organic semiconductors.

Carrier transport and emission efficiency in InGaN quantum-dot based light-emitting diodes

We present a study of blue III-nitride light-emitting diodes (LEDs) with multiple quantum well (MQW) and quantum dot (QD) active regions (ARs), comparing experimental and theoretical results. The LED samples were grown by metalorganic vapor phase epitaxy, utilizing growth interruption in the hydrogen/nitrogen atmosphere and variable reactor pressure to control the AR microstructure. Realistic configuration of the QD AR implied in simulations was directly extracted from HRTEM characterization of the grown QD-based structures. Multi-scale 2D simulations of the carrier transport inside the multiple QD AR have revealed a non-trivial pathway for carrier injection into the dots. Electrons and holes are found to penetrate deep into the multi-layer AR through the gaps between individual QDs and get into the dots via their side edges rather than via top and bottom interfaces. This enables a more homogeneous carrier distribution among the dots situated in different layers than among the laterally uniform quantum well (QWs) in the MQW AR. As a result, a lower turn-on voltage is predicted for QD-based LEDs, as compared to MQW ones. Simulations did not show any remarkable difference in the efficiencies of the MQW and QD-based LEDs, if the same recombination coefficients are utilized, i.e. a similar crystal quality of both types of LED structures is assumed. Measurements of the current-voltage characteristics of LEDs with both kinds of the AR have shown their close similarity, in contrast to theoretical predictions. This implies the conventional assumption of laterally uniform QWs not to be likely an adequate approximation for the carrier transport in MQW LED structures. Optical characterization of MQW and QD-based LEDs has demonstrated that the later ones exhibit a higher efficiency, which could be attributed to better crystal quality of the grown QD-based structures. The difference in the crystal quality explains the recently observed correlation between the growth pressure of LED structures and their efficiency and should be taken into account while further comparing performances of MQW and QD-based LEDs. In contrast to experimental results, our simulations did not reveal any advantages of using QD-based ARs over the MQW ones, if the same recombination constants are assumed for both cases. This fact demonstrates importance of accounting for growth-dependent factors, like crystal quality, which may limit the device performance. Nevertheless, a more uniform carrier injection into multi-layer QD ARs predicted by modeling may serve as the basis for further improvement of LED efficiency by lowering carrier density in individual QDs and, hence, suppressing the Auger recombination losses.

Influence of electromechanical coupling on optical properties of InGaN quantum-dot based light-emitting diodes

The impact of electromechanical coupling on optical properties of light-emitting diodes (LEDs) with InGaN/GaN quantum-dot (QD) active regions is studied by numerical simulations. The structure, i.e. the shape and the average In content of the QDs, has been directly derived from experimental data on out-of-plane strain distribution obtained from the geometric-phase analysis of a high-resolution transmission electron microscopy image of an LED structure grown by metalorganic vapor-phase epitaxy. Using continuum [Formula: see text] calculations, we have studied first the lateral and full electromechanical coupling between the QDs in the active region and its impact on the emission spectrum of a single QD located in the center of the region. Our simulations demonstrate the spectrum to be weakly affected by the coupling despite the strong common strain field induced in the QD active region. Then we analyzed the effect of vertical coupling between vertically stacked QDs as a function of the interdot distance. We have found that QCSE gives rise to a blue-shift of the overall emission spectrum when the interdot distance becomes small enough. Finally, we compared the theoretical spectrum obtained from simulation of the entire active region with an experimental electroluminescence (EL) spectrum. While the theoretical peak emission wavelength of the selected central QD corresponded well to that of the EL spectrum, the width of the latter one was determined by the scatter in the structures of various QDs located in the active region. Good agreement between the simulations and experiment achieved as a whole validates our model based on realistic structure of the QD active region and demonstrates advantages of the applied approach.

Tuning quantum electron and phonon transport in two-dimensional materials by strain engineering: a Green's function based study

The electron and phonon transport properties can be tuned by strain engineering of the transport setup (contact–device–contact).

Role of Ferroelectric Nanodomains in the Transport Properties of Perovskite Solar Cells

Metropolis Monte Carlo simulations are used to construct minimal energy configurations by electrostatic coupling of rotating dipoles associated with each unit cell of a perovskite CH3NH3PbI3 crystal. Short-range antiferroelectric order is found, whereas at scales of 8-10 nm, we observe the formation of nanodomains, strongly influencing the electrostatics of the device. The models are coupled to drift-diffusion simulations to study the actual role of nanodomains in the I-V characteristics, especially focusing on charge separation and recombination losses. We demonstrate that holes and electrons separate into different nanodomains following different current pathways. From our analysis we can conclude that even antiferroelectric ordering can ultimately lead to an increase of photoconversion efficiencies thanks to a decrease of trap-assisted recombination losses and the formation of good current percolation patterns along domain edges.

Efficiency Drop in Green InGaN/GaN Light Emitting Diodes: The Role of Random Alloy Fluctuations

White light emitting diodes (LEDs) based on III-nitride InGaN/GaN quantum wells currently offer the highest overall efficiency for solid state lighting applications. Although current phosphor-converted white LEDs have high electricity-to-light conversion efficiencies, it has been recently pointed out that the full potential of solid state lighting could be exploited only by color mixing approaches without employing phosphor-based wavelength conversion. Such an approach requires direct emitting LEDs of different colors, including, in particular, the green-yellow range of the visible spectrum. This range, however, suffers from a systematic drop in efficiency, known as the "green gap," whose physical origin has not been understood completely so far. In this work, we show by atomistic simulations that a consistent part of the green gap in c-plane InGaN/GaN-based light emitting diodes may be attributed to a decrease in the radiative recombination coefficient with increasing indium content due to random fluctuations of the indium concentration naturally present in any InGaN alloy.

Model of a realistic InP surface quantum dot extrapolated from atomic force microscopy results

We report on numerical simulations of a zincblende InP surface quantum dot (QD) on In₀.₄₈Ga₀.₅₂ buffer. Our model is strictly based on experimental structures, since we extrapolated a three-dimensional dot directly by atomic force microscopy results. Continuum electromechanical, [Formula: see text] bandstructure and optical calculations are presented for this realistic structure, together with benchmark calculations for a lens-shape QD with the same radius and height of the extrapolated dot. Interesting similarities and differences are shown by comparing the results obtained with the two different structures, leading to the conclusion that the use of a more realistic structure can provide significant improvements in the modeling of QDs fact, the remarkable splitting for the electron p-like levels of the extrapolated dot seems to prove that a realistic experimental structure can reproduce the right symmetry and a correct splitting usually given by atomistic calculations even within the multiband [Formula: see text] approach. Moreover, the energy levels and the symmetry of the holes are strongly dependent on the shape of the dot. In particular, as far as we know, their wave function symmetries do not seem to resemble to any results previously obtained with simulations of zincblende ideal structures, such as lenses or truncated pyramids. The magnitude of the oscillator strengths is also strongly dependent on the shape of the dot, showing a lower intensity for the extrapolated dot, especially for the transition between the electrons and holes ground state, as a result of a relevant reduction of the wave functions overlap. We also compare an experimental photoluminescence spectrum measured on an homogeneous sample containing about 60 dots with a numerical ensemble average derived from single dot calculations. The broader energy range of the numerical spectrum motivated us to perform further verifications, which have clarified some aspects of the experimental results and helped us to develop a suitable model for the spectrum, by assuming a not equiprobable weight from each dot, a model which is extremely consistent with the experimental data.

Strong overtones modes in inelastic electron tunneling spectroscopy with cross-conjugated molecules: a prediction from theory

Cross-conjugated molecules are known to exhibit destructive quantum interference, a property that has recently received considerable attention in single-molecule electronics. Destructive quantum interference can be understood as an antiresonance in the elastic transmission near the Fermi energy and leading to suppressed levels of elastic current. In most theoretical studies, only the elastic contributions to the current are taken into account. In this paper, we study the inelastic contributions to the current in cross-conjugated molecules and find that while the inelastic contribution to the current is larger than for molecules without interference, the overall behavior of the molecule is still dominated by the quantum interference feature. Second, an ongoing challenge for single molecule electronics is understanding and controlling the local geometry at the molecule-surface interface. With this in mind, we investigate a spectroscopic method capable of providing insight into these junctions for cross-conjugated molecules: inelastic electron tunneling spectroscopy (IETS). IETS has the advantage that the molecule interface is probed directly by the tunneling current. Previously, it has been thought that overtones are not observable in IETS. Here, overtones are predicted to be strong and, in some cases, the dominant spectroscopic features. We study the origin of the overtones and find that the interference features in these molecules are the key ingredient. The interference feature is a property of the transmission channels of the π system only, and consequently, in the vicinity of the interference feature, the transmission channels of the σ system and the π system become equally transmissive. This allows for scattering between the different transmission channels, which serves as a pathway to bypass the interference feature. A simple model calculation is able to reproduce the results obtained from atomistic calculations, and we use this to interpret these findings.

Resonant electron heating and molecular phonon cooling in single C60 junctions

We study heating and heat dissipation of a single C(60) molecule in the junction of a scanning tunneling microscope by measuring the electron current required to thermally decompose the fullerene cage. The power for decomposition varies with electron energy and reflects the molecular resonance structure. When the scanning tunneling microscope tip contacts the fullerene the molecule can sustain much larger currents. Transport simulations explain these effects by molecular heating due to resonant electron-phonon coupling and molecular cooling by vibrational decay into the tip upon contact formation.

The Green's Function Density Functional Tight-Binding (gDFTB) Method for Molecular Electronic Conduction

A review is presented of the nonequilibrium Green's function (NEGF) method "gDFTB" for evaluating elastic and inelastic conduction through single molecules employing the density functional tight-binding (DFTB) electronic structure method. This focuses on the possible advantages that DFTB implementations of NEGF have over conventional methods based on density functional theory, including not only the ability to treat large irregular metal-molecule junctions with high nonequilibrium thermal distributions but perhaps also the ability to treat dispersive forces, bond breakage, and open-shell systems and to avoid large band lineup errors. New results are presented indicating that DFTB provides a useful depiction of simple gold-thiol interactions. Symmetry is implemented in DFTB, and the advantages it brings in terms of large savings of computational resources with significant increase in numerical stability are described. The power of DFTB is then harnessed to allow the use of gDFTB as a real-time tool to discover the nature of the forces that control inelastic charge transport through molecules and the role of molecular symmetry in determining both elastic and inelastic transport. Future directions for the development of the method are discussed.

Effectiveness of laparoscopic sleeve gastrectomy (first stage of biliopancreatic diversion with duodenal switch) on co-morbidities in super-obese high-risk patients

BACKGROUND

We evaluated laparoscopic sleeve gastrectomy (LSG) on major co-morbidities (hypertension, type 2 diabetes / impaired glucose tolerance, obstructive sleep apnea syndrome (OSAS) and on American Society of Anesthesiologists (ASA) operative risk score in high-risk super-obese patients undergoing two-stage laparoscopic biliopancreatic diversion with duodenal switch (LBPD-DS).

METHODS

41 super-obese high-risk patients (mean BMI 57.3+/-6.5 kg/m(2), age 44.6+/-9.7 years) were entered into a prospective study (BMI > or = 60, or BMI > or = 50 with at least two severe co-morbidities, no Prader-Willi syndrome, no conversion, minimum follow-up 12 months). 9 patients had BMI > or = 60. 17 patients (41.4%) had OSAS on C-PAP therapy. In 10 patients, at least one intragastric balloon had been positioned and 4 had undergone laparoscopic adjustable gastric banding, all with unsatisfactory results. At surgery, 41.5% were classified ASA 4 and 58.5% as ASA 3 (mean ASA score 3.4+/-0.5). Patients underwent evaluation every 3 months postoperatively and were restaged at 12 months and/or before the second step.

RESULTS

60% of major co-morbidities were cured and 24% improved. Average BMI after 6 and 12 months was 44.5+/-8.1 and 40.8+/-8.5 respectively (mean follow-up 22.2+/-7.1 months). After 12 months, 57.8% of the patients were co-morbidity-free and 31.5% had only one major co-morbid condition. At restaging, 20% of patients were still classified as ASA score 4 (OSAS on C-PAP therapy). 3 patients showed BMI <30 and were co-morbidity-free 12 months after LSG.

CONCLUSIONS

LSG represents a safe and effective procedure to achieve marked weight loss as well as significant reduction of major obesity-related co-morbidities. The procedure reduced the operative risk (ASA score) in super-obese patients undergoing two-stage LBPD-DS.

The symmetry of single-molecule conduction

We introduce the conductance point group which defines the symmetry of single-molecule conduction within the nonequilibrium Green's function formalism. It is shown, either rigorously or to within a very good approximation, to correspond to a molecular-conductance point group defined purely in terms of the properties of the conducting molecule. This enables single-molecule conductivity to be described in terms of key qualitative chemical descriptors that are independent of the nature of the molecule-conductor interfaces. We apply this to demonstrate how symmetry controls the conduction through 1,4-benzenedithiol chemisorbed to gold electrodes as an example system, listing also the molecular-conductance point groups for a range of molecules commonly used in molecular electronics research.

Molecular origins of conduction channels observed in shot-noise measurements

Measurements of shot noise from single molecules have indicated the presence of various conduction channels. We present three descriptions of these channels in molecular terms showing that the number of conduction channels is limited by bottlenecks in the molecule and that the channels can be linked to transmission through different junction states. We introduce molecular-conductance orbitals, which allow the transmission to be separated into contributions from individual orbitals and contributions from interference between pairs of orbitals.

Understanding the inelastic electron-tunneling spectra of alkanedithiols on gold

We present results for a simulated inelastic electron-tunneling spectra (IETS) from calculations using the "gDFTB" code. The geometric and electronic structure is obtained from calculations using a local-basis density-functional scheme, and a nonequilibrium Green's function formalism is employed to deal with the transport aspects of the problem. The calculated spectrum of octanedithiol on gold(111) shows good agreement with experimental results and suggests further details in the assignment of such spectra. We show that some low-energy peaks, unassigned in the experimental spectrum, occur in a region where a number of molecular modes are predicted to be active, suggesting that these modes are the cause of the peaks rather than a matrix signal, as previously postulated. The simulations also reveal the qualitative nature of the processes dominating IETS. It is highly sensitive only to the vibrational motions that occur in the regions of the molecule where there is electron density in the low-voltage conduction channel. This result is illustrated with an examination of the predicted variation of IETS with binding site and alkane chain length.

Laparoscopic splenectomy in the management of benign and malignant hematologic diseases

OBJECTIVES

The use of laparoscopy to treat malignant hematological diseases is not completely accepted. Our aim was to analyze operative and postoperative results of laparoscopic splenectomy performed for benign versus malignant hematological disorders.

METHODS

Between 1994 and 2003, 76 consecutive patients underwent laparoscopic splenectomy. The first 38 cases were performed by using an anterior approach, whereas in the remaining 38 cases a semilateral position was used.

RESULTS

Baseline characteristics showed that patients with malignant diseases were significantly older (56.9 vs 32.6 years, P < 0.001). Seventy-two (94.7%) procedures were completed laparoscopically. Conversion was required in 4 cases (5.2%). Mean operative time was 138.5 minutes for benign and 151.0 minutes for malignant diseases, (P > 0.05, ns). The hand-assisted technique was used in 3 patients with massive splenomegaly. Pathologic features showed that spleen volume was higher in patients with malignant diseases (mean interpole diameter 18.1 cm vs 13.7 cm, P < 0.001). Massive splenomegaly (interpole diameter over 20 cm, weight over 1000 g) was present in 13 patients (17.1%); 9 had malignant diseases. Overall perioperative mortality was 1.3% and major postoperative complications occurred in 6 patients (7.8%). Postoperative splenoportal partial thrombosis was identified in 9.7% of patients.

CONCLUSIONS

Laparoscopic splenectomy is a well-accepted, less-invasive procedure for hematological disorders. Neoplastic diseases or splenomegaly, or both, do not seem to limit the indications for a minimally invasive approach after the learning curve.

Prevalence of cancer in Italian obese patients referred for bariatric surgery

BACKGROUND

An association between obesity and cancer has been shown in large epidemiological studies. The aim of this study was to evaluate the prevalence and types of malignancies in an Italian cohort of obese patients referred to a bariatric center.

METHODS

A retrospective, observational study was conducted. Between Jan 1996 and Dec 2004, 1,333 obese patients (M=369, F=964) were seen in the center for minimally invasive treatment of morbid obesity. Morbid obesity were considered as BMI >40 kg/m(2) or BMI >35 kg/m(2) with at least one co-morbidity. Obese and morbidly obese patients who suffered any form of cancer were reviewed.

RESULTS

43 patients (3.2%) presented various malignancies, with 88.3% in females. The prevalence of cancer in the younger group (21-46 years) was higher than in the older group (47-70 years), 2.1% vs 1.1%. 26 obese patients out of the 43 (60.5%) (age 41+/-7.9 years, BMI 38.2+/-9.9) presented hormone-related tumors. The most frequent site of cancer was breast (20.9%), followed closely by thyroid.

CONCLUSION

This is the first Italian report on prevalence of cancer in a homogeneous obese population attending an academic bariatric center. The morbidly obese patients appear to have a higher risk of developing cancer, with a higher prevalence of hormone-related tumors. The predominant gender affected by both obesity and cancer was female. Thus, a preoperative work-up for cancer screening is indicated in this group of patients.

Results after laparoscopic adjustable gastric banding in patients over 55 years of age

BACKGROUND

Laparoscopic adjustable gastric banding (LAGB) has become the most popular bariatric intervention in Europe. International guidelines recommended age limits for bariatric surgery of 18-60 years. The aim of this study was to evaluate the immediate results in morbidly obese patients >55 years old, treated with LAGB.

METHODS

Between January 1996 and January 2004, 350 patients underwent LAGB. 24 (6.8%) were >55 years old (Group A), mean age 58.6+/-3.3 years, mean preoperative BMI 42.3+/-4.5 kg/m2. A comparative randomized analysis with 24 patients younger than age 55 years was performed (Group B: mean age 41.2+/-9.6 years, mean BMI 42.1+/-3.6 kg/m2). Baseline clinical features, operative parameters and postoperative results were evaluated.

RESULTS

No perioperative complications were recorded. Conversion rate and mortality were nil. Major postoperative complications occurred in 2 patients (8.3%) from Group A (1 intragastric prosthesis migration, 1 pouch dilatation) and 2 patients (8.3%) from Group B (intragastric migrations). Reoperation was needed in 3 cases, and one erosion (Group B) is on the waiting list for removal. Minor complications: 1 port infection in each group required ambulatory port substitution; 1 intraperitoneal portcatheter disconnection (Group B) was successfully treated laparoscopically. Mean follow-up was 31.7 months (Group A) and 33.0 months (Group B). Mean postoperative BMI at 12 and 24 months was 35.9+/-4.2 and 33.8+/-4.9 for Group A, and 33.8+/-4.6 and 33.2+/-6.0 for Group B.

CONCLUSION

There have been no significant differences in results between the 2 groups. LAGB has been safe and effective in patients >55 years old.

Role of a Minimally Invasive Approach in the Management of Laparoscopic Adjustable Gastric Banding Postoperative Complications

HYPOTHESIS

Complications after laparoscopic adjustable gastric banding as treatment for morbid obesity may require a major reintervention. A minimally invasive approach represents an attractive management alternative for such complications.

DESIGN

Prospective case series.

SETTING

Major academic medical and surgical center.

PATIENTS

From January 1996 to July 2003, 47 patients who had undergone laparoscopic adjustable gastric banding were operated on again. Considering the causes for reoperation, the patients were divided into 4 groups: group A had major complications (n = 26); group B, minor complications (n= 11); group C, psychological problems (n=6); and group D, insufficient weight loss (n=4).

INTERVENTIONS

Forty-three procedures, 38 using general anesthesia (groups A, C, and D) and 5 using local anesthesia (group B), were performed.

MAIN OUTCOME MEASURES

Feasibility, safety, and effectiveness of a minimally invasive approach in the treatment of laparoscopic adjustable gastric banding complications.

RESULTS

In group A, 9 of 10 patients with irreversible gastric pouch dilatation and 15 of 16 with intragastric band migrations were treated laparoscopically. In group B, 5 ports were substituted and 2 reconnections of the catheter-port system were performed. In group C, 6 laparoscopic band removals were carried out. In group D, 4 laparoscopic revision procedures for insufficient weight loss were performed. The operative mortality was nil. The most frequent cause of reoperation was intragastric migration (37.2%). A minimally invasive approach was adopted in 94.7% of cases.

CONCLUSION

Laparoscopy is safe and effective, even as a second operative procedure.

Tailorable acceptor C(60-n)B(n) and donor C(60-m)N(m) pairs for molecular electronics

Our first-principles calculations demonstrate that C(60-n)B(n) and C(60-m)N(m) can be engineered as the acceptors and donors, respectively, needed for molecular electronics by properly controlling the dopant number n and m in C60. We show that acceptor C48B12 and donor C48N12 are promising components for molecular rectifiers, carbon nanotube-based n-p-n (p-n-p) transistors, and p-n junctions.

[Laparoscopic splenectomy: analysis of 60 consecutive cases]

The purpose of the study was to analyze the results of 60 patients who were candidates for laparoscopic splenectomy. Over the period from May 1994 to May 2001, 60 patients were candidates for splenectomy. Laparoscopy was contraindicated in 3 cases because of ASA III and marked splenomegaly (2 cases) and previous gastric resection (1 case). The procedure was indicated for benign disease in 38 cases and for malignant disease in the remainder. Fifty-three procedures were completed laparoscopically (92.9%). Conversion proved necessary in 4 patients (6.7%) due to large incisional hernia, perisplenic abscess, bleeding of major splenic vessels at the hilum and marked splenomegaly (2 cases of lymphoma). The mean operative time was 200 min for the malignancies and 110 min for the benign conditions (P < 0.05). Major morbidity occurred in 5 cases (8.7%). No deaths were registered. The mean postoperative hospital stay was 7.5 days for patients with malignancies and 5.2 days for patients with benign disease (P < 0.05). Laparoscopic splenectomy was safe and effective in patients with benign disease, even in cases of marked splenomegaly. The morbidity rate was significantly higher in lymphoma patients than in patients with benign haematological disorders.

Structure-activity relationships in cinnamamides. 1. Synthesis and pharmacological evaluation of some (E)- and (Z)-N-alkyl-alpha,beta-dimethylcinnamamides

Two series of (E)- and (Z)-N-alkyl-alpha,beta-dimethylcinnamamide derivatives were prepared and the biological activity of these compounds was investigated in a series of pharmacological tests. All compounds tested had clear activity on the CNS; generally, this was depressant with E isomers, while Z isomers always caused marked stimulation (tremors and convulsions). Some of the E isomers also had a clear-cut anticonvulsant activity as shown by the antagonistic effect on pentylenetetrazole-induced seizures in the mouse. The NMR spectra of these compounds, which confirm their configurations, are discussed.